1. Field of the Invention
The present invention relates in general to ignition systems for internal combustion engines and more particularly to an ignition secondary circuit sensor for detecting both of ignition current flowing through a spark plug of an ignition system at the time of spark discharge of the spark plug and ion current flowing through the spark plug at the time of combustion of fuel in a cylinder of the engine. The present invention further relates to a device for detecting an ignition timing and a combustion timing of an internal combustion engine by the use of the above described ignition secondary circuit sensor. The present invention further relates to an apparatus for detecting pre-ignition of an internal combustion engine on the basis of an ignition timing and a combustion timing detected by the above described ignition timing-and-combustion timing detecting device.
2. Description of the Related Art
The above described ignition secondary circuit sensor is known as for example disclosed in Japanese patent provisional publication No. 4-308362. The sensor has a detection path connected to a detecting circuit. The detection path is capacitively coupled with a high voltage path of an ignition system so that the sensor can detect ignition current flowing through the high voltage path in addition to ion current. Due to this, after spark discharge of a spark plug, reverse current which flows through the high voltage path in the reverse direction to the ignition current is caused to flow into the detecting circuit by way of a capacitive coupling portion, thus causing a problem that the detecting circuit erroneously judges the reverse current as ion current.
Namely, at the time of spark discharge of the spark plug, ignition current flows through the high voltage path in one direction, i.e., from the spark plug to an ignition circuit side. Thereafter, by an energy stored in a secondary winding of an ignition coil, reverse current is caused to flow through the high voltage path in the reverse direction to the ignition current. Then, currents in one and the other directions flow through the high voltage path alternately until the energy stored in the secondary winding of the ignition coil is discharged completely. The detecting circuit erroneously judges the current flowing through the high voltage path in the reverse direction to the ignition current as ion current.
Another problem is that erroneous detection of an ignition timing and ion current results from noise or the like since the noise caused on the ignition secondary circuit side of the ignition system is inputted to the detecting circuit by way of the capacitive coupling portion.